He does have sort of a straight take back, but he is hitting with a western or semi-western grip and it looks like he takes his wrist back as far as he wants to before the take back. It's a little hard to tell from that video how much he's using the wrist on the take back though.

In a few stills I've seen of him he reminds me of McEnroe in his set up on the forehand at times - very straight arm and looking to hit with the shoulder. Supposedly his change of pace is something that disrupts opponents and it may be that he does get some snap into the shot, just not all the time.